A multiplicity of methods and instruments for measuring a target point have been known since antiquity. Here, the distance and angle from a measurement instrument to a target point are recorded as a spatial standard data and, in particular, the location of the measurement instrument is acquired, in addition to possibly present reference points.
A laser tracker, a theodolite, a tachymeter and a total station represent known examples for such measuring instruments, wherein the last example is also referred to as electronic tachymeter or computer tachymeter.
Such instruments comprise electrosensory angle and distance measurement functions, which render it possible to determine a direction and a distance to a selected target. Here, the angle or distance variables are established in the internal reference system of the instrument and may still need to be linked to an external reference system for determining an absolute position.
In general, measurement devices embodied for continuous tracking of a target point and for a coordinative position determination of this point can be subsumed under the term laser tracker, in particular in the context of industrial measurement. Here, a target point can be represented by a retroreflecting unit (e.g. triple prism or corner cube reflector), which is sighted using a directed optical measurement beam of the measurement device, in particular using a laser beam. The laser beam is reflected back to the measurement device in parallel, with the reflected beam being acquired by an acquisition unit of the device. Here, an emission or reception direction of the beam is established, for example by means of sensors for measuring angles, which are assigned to the deflection mirror or a targeting unit of the system. Moreover, a distance from the measurement device to the target point is established when acquiring the beam, for example by means of a time-of-flight of a phase difference measurement or by means of the Fizeau principle.
Moreover, an offset of the received measurement beam from a zero position is established generically at a fine targeting or tracking sensor in tracker systems. A difference in position between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined by means of this measurable offset and the alignment of the laser beam can be corrected or updated dependent on this deviation in such a way that the offset is reduced on the fine targeting sensor, in particular set to “zero”, and therefore the beam is aligned in the direction of the reflector center. By updating the laser beam alignment there can be continuous target tracking of the target point and the distance and position of the target point relative to the measuring instrument can be determined continuously. Here, updating can be realized by means of a change in alignment of the deflection mirror which is movable by motor and provided for deflecting the laser beam, and/or by swiveling the targeting unit which has the beam-guiding laser optics.
Laser trackers according to the prior art may additionally be configured with one or more optical image acquisition units with a two-dimensional, light-sensitive array, e.g. a CCD or CID, or a camera based on a CMOS array, or with a pixel array sensor and with an image processing unit.
Such cameras—generally with a broad viewing angle compared to the reception optics for the reflected measurement radiation—can e.g. be attached and provided for providing target search functionalities and/or 6-DOF determination functionalities (the latter in conjunction with the use of an e.g. sensing measurement aid).
By acquiring and evaluating an image—by means of image acquisition unit and image processing unit—from a so-called auxiliary measurement instrument with markings, the relative locations of which with respect to one another are known, it is possible to deduce an orientation in space of an object arranged at the auxiliary measurement instrument. Together with the determined spatial position of the target point (e.g. a retroreflector at the auxiliary measurement instrument sighted by the laser beam), it is ultimately furthermore possible to determine precisely the position and orientation of the object in space in absolute terms and/or relative to the laser tracker. Such a coordinate measuring machine with a laser tracker and an image acquisition unit for determining position and orientation of objects in space, at which light points and reflectors are arranged, is described e.g. in U.S. Pat. No. 5,973,788.
Such auxiliary measurement instruments can be so-called sensing tools which, with the contact point thereof, are positioned on a point of the target object. The sensing tool has markings, e.g. light points, and a reflector, which represents a target point at the sensing tool and which can be sighted by the laser beam of the tracker, wherein the positions of the markings and of the reflector are known precisely relative to the contact point of the sensing tool. In a manner known to a person skilled in the art, the auxiliary measurement instrument can also be an e.g. hand-held scanner, which is equipped for point measurements, for a contactless surface measurement, wherein direction and position of the scanner measurement beam used for the distance measurement are accurately known relative to the light points and reflectors arranged on the scanner. By way of example, such a scanner is described in EP 0 553 266.
For measuring the distance, laser trackers according to the prior art have at least one rangefinder, wherein the latter can be embodied e.g. as an interferometer and/or as an absolute rangefinder (e.g. based on time-of-flight or phase difference measurements or the Fizeau principle).
WO 2007/079600 A1 discloses a generic laser-based coordinate measuring machine, in which light emergence and light reception optics of the distance measuring device, a measurement camera and an overview camera are arranged on a common element rotatable in relation to at least two axes and in which a laser beam from a laser module attached outside of the beam guiding unit is coupled into the distance measurement device by means of an optical waveguide.
Within the scope of ongoing development of technologically highly developed measurement systems, which have a very high accuracy potential, strategies for correcting atmospherically induced disturbances become ever more important. These atmospherically induced disturbances are produced over a large timescale range: systematic deviations, which are referred to as “refraction” in the geodetic context and which are caused by a refractive index gradient in the observation surroundings, eventually merge into stochastic deviations, which are caused by optical turbulence or which are equivalent to the effect of optical turbulence, at least in terms of the consequence thereof.
Refraction-corrected optical direction and angle measurements, but also refraction-corrected optical distance measurements, are required within the scope of numerous task areas in the field of highly precise measurement. By way of example, these task areas contain measurement applications not only within the scope of industrial measurement (e.g. in industrial quality-control or assembly processes, e.g. in the aeronautical or automotive industry) but also in the field of geodetic measurement, in the field of structural and civil engineering projects, in the context of alignment problems, e.g. in the case of particle accelerators, or for the spatial control of large moving machines.